U.S. patent number 4,330,574 [Application Number 06/191,857] was granted by the patent office on 1982-05-18 for finishing method for conventional hot dip coating of a ferrous base metal strip with a molten coating metal.
This patent grant is currently assigned to Armco Inc.. Invention is credited to Charles Flinchum, Marvin B. Pierson.
United States Patent |
4,330,574 |
Pierson , et al. |
May 18, 1982 |
**Please see images for:
( Certificate of Correction ) ( Reexamination Certificate
) ** |
Finishing method for conventional hot dip coating of a ferrous base
metal strip with a molten coating metal
Abstract
A finishing method and apparatus for conventional continuous
hot-dip coating of the type wherein a ferrous base metal strip is
caused to pass beneath the surface of a coating bath of molten
coating metal and is thereafter subjected to jet finishing, the
ferrous base metal strip having been appropriately pretreated so as
to be at the proper coating temperature and so as to have its
surfaces oxide-free when passing through the bath of molten coating
metal. The method comprises the steps of providing an enclosure for
the two-side coated strip as it exits the coating bath, locating a
finishing jet nozzle to either side of the coated strip within the
enclosure, jet finishing the coated strip with a non-oxidizing or
inert gas. The apparatus comprises the above mentioned enclosure
with the jet finishing nozzles located therein and an appropriate
system to provide a non-oxidizing or inert atmosphere within the
enclosure.
Inventors: |
Pierson; Marvin B. (Franklin,
OH), Flinchum; Charles (Hamilton, OH) |
Assignee: |
Armco Inc. (Middletown,
OH)
|
Family
ID: |
26706301 |
Appl.
No.: |
06/191,857 |
Filed: |
September 29, 1980 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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30660 |
Apr 16, 1979 |
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Current U.S.
Class: |
427/319; 427/320;
427/321; 427/349; 427/432; 427/433; 427/434.2 |
Current CPC
Class: |
C23C
2/40 (20130101); C23C 2/20 (20130101) |
Current International
Class: |
C23C
2/20 (20060101); C23C 2/20 (20060101); C23C
2/36 (20060101); C23C 2/36 (20060101); C23C
2/14 (20060101); C23C 2/14 (20060101); C23C
2/40 (20060101); C23C 2/40 (20060101); B05D
003/02 () |
Field of
Search: |
;427/320,321,329,432,374.4,433,319,434.5,434.6,434.2,434.7,434.4,345,349
;118/63,65 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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55-62154 |
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May 1980 |
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JP |
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588281 |
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May 1947 |
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GB |
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Primary Examiner: Morgenstern; Norman
Assistant Examiner: Childs; S. L.
Attorney, Agent or Firm: Frost & Jacobs
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This is a continuation-in-part of application Ser. No. 30,660,
filed Apr. 16, 1979, now abandoned, in the name of the same
inventors and titled FINISHING METHOD AND APPARATUS FOR
CONVENTIONAL HOT DIP COATING OF A FERROUS BASE METAL STRIP WITH A
MOLTEN COATING METAL.
Claims
What is claimed is:
1. A finishing process for conventional continuous hot-dip,
two-side coating of a ferrous base metal strip with a molten
coating metal of the type wherein said ferrous base metal strip is
caused to enter a bath of said molten coating metal contained in a
coating pot, said ferrous base metal strip having been treated to
bring it to a coating temperature sufficiently high to prevent
casting of said coating metal thereon and low enough to prevent
excess coating metal-base metal alloying and to render the surfaces
of said strip clean and free of oxide as it passes through said
molten coating metal bath, said finishing process comprising the
steps of providing an enclosure in sealed relationship with said
bath for said two-side coated ferrous base metal strip as it exits
said bath and an exit in said enclosure for said coated strip,
maintaining a non-oxidizing atmosphere within said enclosure,
locating a jet finishing nozzle to either side of said coated strip
within said enclosure, jet finishing said coated strip with a
non-oxidizing gas and maintaining said jet finishing gas and said
atmosphere within said enclosure at an oxygen level of less than
about 1000 ppm, whereby to render said two-side coated strip
uniform in appearance and coating thickness.
2. The process claimed in claim 1 wherein said molten coating metal
is chosen from the class consisting of zinc, zinc alloys, aluminum,
aluminum alloys, terne and lead.
3. The process claimed in claim 1 including the step of maintaining
said jet finishing gas and said atmosphere within said enclosure at
an oxygen level of less than about 200 ppm.
4. The process claimed in claim 1 including the step of maintaining
said jet finishing gas and said atmosphere within said enclosure at
an oxygen level of less than about 100 ppm.
5. The process claimed in claim 1 wherein said non-oxidizing jet
finishing gas and said atmosphere within said enclosure comprise
nitrogen.
6. The process claimed in claim 1 wherein said non-oxidizing jet
finishing gas and said atmosphere within said enclosure comprise an
inert gas.
7. The process claimed in claim 1 including the step of vertically
staggering said jet finishing nozzle with respect to each
other.
8. The process claimed in claim 1 wherein each of said jet
finishing nozzles has a rectangular nozzle opening of uniform width
throughout its length and equidistant from said coated strip
throughout its length.
9. The process claimed in claim 1 including the step of
recirculating a portion of said non-oxidizing gas from said
enclosure through said jet finishing nozzles.
10. The process claimed in claim 1 wherein said molten coating
metal is chosen from the class consisting of zinc and zinc alloys
and including the step subjecting said two-side coated strip to a
spangle minimizing treatment after said strip exits said
enclosure.
11. The process claimed in claim 5 including the step of
recirculating a portion of said nitrogen from said enclosure
through said jet finishing nozzles.
12. The process claimed in claim 5 including the step of vertically
staggering said jet finishing nozzles with respect to each
other.
13. The process claimed in claim 12 wherein each of said jet
finishing nozzles has a rectangular nozzle opening of uniform width
throughout its length and equidistant from said coated strip
throughout its length.
14. The process claimed in claim 13 including the step of
maintaining said nitrogen at an oxygen level of less than about 100
ppm.
15. The process claimed in claim 14 including the step of
subjecting said two-side coated strip to a spangle minimizing
treatment after said strip exits said enclosure.
Description
TECHNICAL FIELD
The invention relates to a finishing method and apparatus for
conventional continuous hot-dip coating of a ferrous base metal
strip with a molten coating metal, and more particularly to a
method and apparatus whereby the coated ferrous base metal strip,
upon exiting the coating bath, is maintained in an essentially
oxygen-free atmosphere until jet finished with a non-oxidizing or
inert gas.
BACKGROUND ART
The method and the apparatus of the present invention are
applicable to a hot-dip coating of a ferrous base metal strip with
zinc, zinc alloys, aluminum, aluminum alloys, terne, lead and those
coating metals or coating metal alloys which have oxide forming
characteristics such that optimum finishing cannot be accomplished
by conventional jet practice or by conventional exit rolls. While
not intended to be so limited, for purposes of an exemplary showing
the method of the present invention will be described as applied to
galvanizing. The method can be practiced on various types of
galvanizing lines. For example, the method of the present invention
is applicable to fluxless, hot-dip metallic coating of ferrous base
metal strip where it is necessary to subject the strip surfaces to
a preliminary treatment which provides the strip with oxide free
surfaces and preferably brings the strip to a temperature
approximating that of the molten zinc or zinc alloy coating bath at
the time the strip is caused to pass beneath the surface thereof.
One of the principal types of anneal-in-line, fluxless, preliminary
treatments is the so-called Sendzimir process or
oxidation-reduction practice disclosed in U.S. Pat. Nos. 2,110,893
and 2,197,622. Another anneal-in-line, fluxless, preliminary
treatment in common use is the so-called Selas process or high
intensity direct fired furnace practice disclosed in U.S. Pat. No.
3,320,085.
In the Selas process the ferrous base metal strip is passed through
a direct fired preheat furnace section. The strip is heated by
direct combustion of fuel and air producing gaseous products of
combustion containing at least about 3% combustibles in the form of
carbon monoxide and hydrogen. The strip reaches a temperature of
about 535.degree. C. to about 760.degree. C. while maintaining
bright surfaces completely free of oxidation. The strip is then
passed into a reducing section which is in sealed relation to the
preheat section and which contains a hydrogen and nitrogen
atmosphere, wherein it may be further heated by radiant tubes to
from about 650.degree. C. to about 925.degree. C. and thereafter
cooled approximately to the molten coated metal bath temperature.
The strip is then led beneath the bath surface while surrounded by
the protective atmosphere.
Other related pretreatment techniques are taught in U.S. Pat. No.
Re 29, 726--U.S. Pat. Nos. 3,837,790--4,123,291--4,123,292 and
4,140,552. The above mentioned prior art patents constitute
non-limiting examples of fluxless, continuous galvanizing processes
to which the method of the present invention is applicable. When
such conventional strip preparation techniques as are taught in the
above mentioned prior art are used, it is necessary that the base
metal strip be maintained in a protective atmosphere at least until
it passes beneath the surface of the bath of molten zinc or zinc
alloy.
Such a protective atmosphere is not a requirement when flux or
chemical strip preparation techniques of the type taught in U.S.
Pat. Nos. 2,824,020 and 2,824,021 are employed. Briefly, when such
chemical strip preparation techniques are used, the ferrous base
metal strip is caused to pass through a flux bath and through means
to assure the proper thickness of the flux coating on the strip.
The ferrous base metal strip is then conducted through a heating
chamber wherein the strip is heated to evaporate the water in the
flux solution. Thereafter, the ferrous base metal strip is further
heated to raise it to that temperature approaching the maximum
temperature of stability of the flux coating on the strip. The
strip is then caused to pass beneath the surface of the bath of
molten zinc or zinc alloy so as to be coated. The method of the
present invention is equally applicable to galvanizing lines
utilizing such flux or chemical pretreatment systems.
From the above it will be evident that the method of the present
invention is not limited to the use of any particular pretreatment
of the ferrous base metal strip in the galvanizing line and the
terms "pretreatment" or "pretreated" (as used herein and in the
claims with reference to the ferrous base metal strip) are to be
interpreted broadly to include any of the conventional pretreatment
systems exemplified by the above noted prior art. In general, these
terms refer to any appropriate pretreatment technique, the result
of which is such that, during the actual coating step wherein the
ferrous base metal strip passes through the molten bath of zinc or
zinc alloy, it will be at or will achieve the proper coating
temperature and its surface will be oxide-free.
In conventional continuous hot dip galvanizing, it is usual to
cause the two-side coated strip to exit the molten coating metal
bath into the ambient atmosphere. The most widely used finishing
and coating weight control technique is to direct the coated strip
between jet knives or nozzles which cause a blast of air or steam
to impinge upon both sides of the coated strip, returning excess
coating metal to the bath. This finishing technique, however, has a
number of definite drawbacks. Some of these drawbacks include
coating ripples, oxide curtains and, bath surface oxide problems,
including the necessity for top skimming. All of these defects are
real, but are reasonably controllable by known methods.
U.S. Pat. Nos. 4,107,357 and 4,114,563 and German Pat. No.
2,656,524 are exemplary of patents teaching methods for coating one
side only of a ferrous base metal strip. In the practice of these
processes, the coated strip after contact with the coating bath is
maintained in a protective, non-oxidizing atmosphere and is jet
finished with nitrogen or non-oxidizing gas. However, the primary
purpose of these steps is to prevent oxidation of that side of the
ferrous base metal strip not coated or, if the uncoated side has an
oxide film thereon, to prevent adherence of the coating metal to
the oxide film. Nevertheless, U.S. Pat. No. 4,114,563 mentions in
passing that the above noted defects are controlled or eliminated
when finishing in a protective atmosphere.
A major problem area encountered with conventional jet finishing is
that of coating control at the edges of the strip. One edge problem
is that of zinc coating thickness over a narrow band immediately
adjacent each edge of the coated strip. The coating thickness of
these bands is greater than the coating thickness over the rest of
the strip width. If this coating thickness differential is great
enough, edge buildup or spooling will occur when the continuous
strip is coiled under tension.
Edge berries (small balls of oxide) which are attached to the strip
edge and are pulled through the jet blast constitute another
troublesome problem. Furthermore, an edge defect commonly known as
"feathered oxide" occurs during low speed jet finishing. Feathered
oxide is characterized by discontinuous patches of heavy coating
metal oxide which pull through the jet blast. They appear much like
feathers which extend inwardly from the strip edges with the tips
thereof pointing toward the center of the strip.
Many methods have been used by prior art workers to reduce build-up
and oxide control problems at the strip edges. Tapered jet nozzle
slot openings are commonly used where the slot opening of the jet
finishing nozzle continuously increases in width from the center of
the jet nozzle to its ends. Such a contoured jet finishing nozzle
is taught in U.S. Pat. No. 4,137,347.
Other methods to control edge coating include curving the jet
nozzles so that the nozzle is closer to the strip at the strip
edges than at the strip center. Also, vanes or nozzle extensions
have been used at the strip edges to bring the nozzle closer to the
edges than to the center of the strip. Still other methods include
the use of shutters and auxiliary jets, both internal and external
to the main jet nozzles, to alter the jet wiping force at the strip
edges as compared to the jet wiping force at the strip center.
All prior art methods fall short of producing optimum edge control
with a minumum of operator attention, maximum coating metal
economy, and proper edge control over a wide range of strip widths
and line speeds.
Edge build up in a one-side coating process differs from that
encountered in a conventional two-side coating process. This is
true because, when jet finishing is employed, it is normally
performed with a single jet nozzle, preferably with a back-up roll
on the uncoated side of the strip. As a result, edge build-up is
less severe, since the back-up roll serves as an extension of the
strip edges.
Yet another major hot-dip zinc coating defect is commonly referred
to as "spangle relief". Spangle relief has two aspects. One is the
variation of surface profile (zinc thickness) across the zinc
crystal from one boundary to the opposite boundary. The other is a
depressed spangle boundary which surrounds each spangle or crystal.
Spangle relief can be reduced by such methods as purposely causing
part of the zinc coating to alloy with the ferrous base metal, or
by antimony additions to the zinc bath. However, none of these
methods is entirely satisfactory.
As a result, many methods have been developed to suppress spangle
formation. That is, to minimize final spangle size to such an
extent that the spangles are hardly visible to the naked eye. For
example, U.S. Pat. Nos. 3,322,558; 3,379,557 and 3,756,844 teach
spangle minimizing methods. Most methods involve spraying water or
water solutions against the molten coating to quench the coating
and create many nucleation sites. However, the results achieved by
these spangle minimizing methods are not always consistent.
While spangle relief can be reduced or masked by temper (skin-pass)
rolling such temper rolling causes these defects to become
imprinted in the base metal. As a result of this non-uniform cold
working of the base metal, the defects may reappear when critical
surface items such as automotive body parts and appliance parts are
stamped or formed. These defects may not be completely masked after
the parts are painted.
Coating irregularity problems constituted a major reason why prior
art workers have recently turned their attention to a one-side
coated product (as exemplified by U.S. Pat. No. 4,082,868) wherein
the uncoated side is the side to be painted on critical surface
items made therefrom, even at the sacrifice of corrosion resistance
on the non-galvanized side.
Yet another problem area encountered with conventional jet
finishing involves coating weights and line speeds. The viscous
interaction between the coating metal and the strip is proportional
to strip speed. At slow speeds, the prior art was faced with the
problem of ripple formation. To combat this, it was found that
minimizing jet nozzle to strip distance and reducing the jet
finishing pressure will minimize the prominence of coating ripples.
However, low jet finishing pressure and close positioning of the
jet finishing nozzles at the same time create an edge build-up
problem. Prior art workers therefore have had to adjust the
parameters to control edge build-up and ripples and this has
necessitated higher line speeds. As an example, it has been common
practice to use conventional jet finishing only with strip speeds
above about 100 feet per minute (30 meters per minute) to produce
commercial class coating weight (ASTM A525, G90). Edge build-up
problems on G-90 coating commonly occur at speeds below about 150
feet per minute (45 meters per minute). Heavier coatings are even
more difficult to control edge-to-edge uniformly.
Another essential practice in most prior art jet finishing
operations is to position the jet nozzles virtually directly
opposed, such that the jet streams are in direct interference
beyond the edges of the strip. This interference results in
extremely high and objectionable noise levels. Vertically offset
jet nozzles cause a wrap-around effect with a bead of heavy coating
metal along the edge on the side opposite the last nozzle operating
on the strip. In addition to the noise problem and the need for
precise adjustment of the jet nozzles by the operator, opposed
operation can result in coating metal splatter being blown off of
the strip edge by one nozzle and into the nozzle opening of the
opposed nozzle.
Prior art workers have hitherto jet finished hot-dipped, two-side
coated galvanized and aluminized strip with nitrogen. Such jet
finishing, however, has been performed in an ambient atmosphere. In
jet finishing, less nitrogen is required when nitrogen is used than
air when air is used. However, the results achieved by such
finishing are more nearly like those achieved in jet finishing with
air in an ambient atmosphere than like the results achieved by the
present method.
U.S. Pat. Nos. 3,505,042 and 3,505,043 teach hot dip coating of a
ferrous metal with magnesium-zinc alloy and magnesium-aluminum-zinc
alloy respectively. The first of these references utilized coating
rolls for coating control and nitrogen gas solely for cooling after
finishing between exit rolls. The second of these references does
not disclose specific finishing means, but does enclose the coating
apparatus in an atmosphere of 10% hydrogen with the balance
nitrogen. The reference also mentions blowing nitrogen gas over the
coated strip being withdrawn from the bath to rapidly cool the
coating and to control coating weight, the nitrogen gas having a
temperature below 100.degree. F. (40.degree. C.) and preferably
about 50.degree. F. (10.degree. C.).
The present invention is based upon the discovery that if, in a
conventional, continuous, hot-dip, two-side galvanizing process,
the coating metal as it exits the coating bath is surrounded by an
enclosure in which a substantially oxygen-free atmosphere is
maintained and if within the enclosure the coated strip is jet
finished with non-oxidizing or inert gas, the finishing problems
encountered with conventional finishing methods are markedly
reduced or eliminated. The finishing method of the present
invention enables the practice of any of the above noted spangle
minimizing techniques, greatly enhancing the results achieved and
the uniformity of spangle minimization throughout the width of the
strip. Furthermore, the markedly improved results are
consistent.
The most significant aspect of the present invention, however, is
the discovery that all coating control problems at the strip edges
are completely eliminated with the substantial exclusion of oxygen
from the finishing process. Minimum operating speeds are no longer
limited by edge build-up problems, but rather only by the desired
coating weight relative to the amount of coating metal naturally
pulled up by the strip to the finishing jet nozzles. It has been
found, for example, that excellent quality G-90 coatings can be
produced without difficulty at speeds as low as 30 feet per minute
(9 meters per minute).
Coating control at the strip edges is a very real problem in a
two-side coating process where jet finishing cannot be accomplished
against a back-up roll as in one-side coating processes. In the
practice of the teachings of the present invention, heavy coating
at the strip edges does not occur. Jet nozzles can be vertically
offset, eliminating the need for precise positioning, greatly
reducing noise, and eliminating the hazard of zinc splatter
plugging opposed nozzles. Jet nozzle design may be simplified to
utilize a slot-like nozzle opening of uniform width throughout its
length, rendering unnecessary the multitude of special jet nozzle
designs, methods and accessories which have been used for
controlling edge build-up. Superior uniform coating thickness
results edge-to-edge for all coating weights because the center
profile need no longer be distorted to compensate for heavy edges.
As a result, coils can be wound under high tension without
spooling. Furthermore, with uniform coating weight edge-to-edge,
less coating metal is used. The greatly reduced noise, by virtue of
vertically offsetting the jet nozzles in an enclosure, represents a
major step in safety and environment improvement. The simplified
jet nozzle design results in more uniform coating, more uniform
cooling and a flatter strip.
Heretofore, in the practice of conventional two-side jet finishing,
neither the mechanism for ripple formation nor that causing edge
build-up problems was completely understood. In the process of
conventional jet finishing, a pneumatic dam effect is created
whereby the desired amount of coating metal is metered through the
jet barrier to form the finished coating. At this metering point
the excess coating metal pulled up with the strip, beyond that
required for the finished coating, is returned to the coating bath.
This process is described in detail in U.S. Pat. No. 4,078,103.
While applicants do not wish to be bound by theory, it appears as a
result of the present invention that the coating ripples and heavy
edge coating in conventional jet finishing are caused entirely by
coating metal oxide. At some point in the jet interaction region,
probably just above the point of zero surface velocity , fresh
(unoxidized) coating metal is being exposed and, as it is exposed,
it immediately forms a very light oxide skin. The continuity of
flow or distribution of this very light oxide skin onto the
finished coating determines the occurrence of coating ripples. In
conventional practice, the jet periodically restrains the oxide
film. The film builds up until the jet no longer can restrain it.
At this time a segment of relatively heavy oxide breaks off and
passes with the finished coating. The segment as it passes carries
with it coating beneath which is heavier than that which is metered
on when the oxide film is restrained. This process is repeated many
times each second as ripples are formed.
A similar mechanism is believed to be operable in creating heavy
coating metal along the strip edges. However, at the edges,
geometry becomes an additional important factor in that there is no
wiping force directed against the edge surfaces. Relatively heavy
oxide is permitted to pass through the jet interaction area more or
less continuously carrying with it heavy coating beneath. This
oxide envelope around each strip edge surface is the "container"
which permits zinc wrap-around to occur when the jet nozzles are
vertically offset.
Edge build-up of the molten coating metal is eliminated by the
practice of the present invention by avoiding oxidation.
The zinc coated product produced by the method of the present
invention (including spangle minimization) has such excellent
surface qualities after temper rolling that it is suitable for use
in exposed automotive body panels, appliance applications and the
like.
DISCLOSURE OF THE INVENTION
The invention is directed to an improved finishing method and
apparatus for conventional continuous hot-dip coating lines
producing two-side coated product. As indicated above, for purposes
of an exemplary showing the invention will be described in its
application to a galvanizing line. The conventional continuous
hot-dip galvanizing line may be of any appropriate and well known
type utilizing any suitable pretreatment steps for the ferrous base
metal strip to assure that the surfaces of the ferrous base metal
strip achieve an oxide-free condition and a proper coating
temperature prior to or during its travel through the bath of
molten zinc or zinc alloy.
In accordance with the method of the present invention, the coated
strip exiting the galvanizing bath is maintained in an enclosure
and is subjected to jet finishing within the enclosure. The jet
finishing is accomplished with an inert or non-oxidizing gas and
the jet finishing gas and the atmosphere within the enclosure are
maintained at an oxygen level below 1000 ppm. It is preferred that
the oxygen level in both the enclosure atmosphere and the jet
finishing gas be less than about 200 parts per million and, for
optimum aresults, less than about 100 parts per million. The jet
finishing may be achieved using any appropriate type of jet nozzles
which may be staggered vertically with respect to each other, if
desired. This invention allows the use of nozzles having a
rectangular slot equidistant from the strip throughout its length.
Steps are taken to prevent the entrance of ambient atmosphere into
the enclosure particularly at the point where the coated strip
exits the enclosure. When the coating metal is zinc or zinc alloy,
the finishing step may be followed by a conventional spangle
minimizing step conducted outside the enclosure.
The enclosure of the present invention is configured to maintain an
inert or non-oxidizing atmosphere about the strip exiting the bath
of molten coating metal. The enclosure houses the jet nozzles. The
enclosure can be a separate structure or an integral part of the
snout through which the strip enters the bath of molten coating
metal. Various coating bath roll arrangements can be used, as will
be described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a fragmentary, semi-diagrammatic, cross sectional
elevational view of an exemplary continuous hot-dip galvanizing
line equipped to practice the method of the present invention.
FIG. 2 is an enlarged, fragmentary, semi-diagrammatic, cross
sectional view of the coating end of the galvanizing line of FIG.
1.
FIGS. 3, 4 and 5 are enlarged, fragmentary, semi-diagrammatic cross
sectional views similar to FIG. 2, but illustrating various pot
roll arrangements.
FIG. 6 is a fragmentary, semi-diagrammatic cross sectional view
illustrating the enclosure of the present invention as constituting
an integral part of the snout through which the strip enters the
bath of molten coating metal.
FIG. 7 is a fragmentary, semi-diagrammatic cross sectional view,
similar to FIG. 6, but showing a partially submerged pot roll and
the use of a pump for the molten coating metal.
DETAILED DESCRIPTION OF THE INVENTION
While not intended to be so limited, for purposes of an exemplary
showing the method of the present invention will be described as
applied to a Selas-Type galvanizing line. Turning to FIG. 1, the
coating line is generally indicated at 1. The strip preparation
furnace of the coating line comprises a direct fired furnace 2, a
controlled atmosphere heating furnace 3, a first cooling section 4,
a second cooling section 5 and a snout 6. It will be noted that the
snout 6 is configured to extend below the upper surface of a bath 7
of molten coating zinc or zinc alloy, located in a coating pot
8.
The ferrous base metal strip 9 to be prepared enters the direct
fired furnace 2 over rolls 10 and 11 and through sealing rolls 12
and 13, so located as to minimize the escape of products of
combustion through the entrance opening 14 of preheat furnace 2.
The direct fired furnace 2 operates at a temperature on the order
of 1260.degree. C. (2300.degree. F.). The function of the direct
fired furnace is to quickly burn away oil and the like from the
surfaces of the ferrous base metal strip 9, while providing partial
heating for annealing the strip. The direct fired furnace, at the
temperature indicated, will be sufficient to heat the entering
strip to a temperature of from about 535.degree. C. (1000.degree.
F.) to about 760.degree. C. (1400.degree. F.) by the time it passes
from the direct fired furnace to the controlled atmosphere heating
furnace 3.
The ferrous base metal strip 9 passes about turn-around rolls 15
and 16 and begins an upward travel through controlled atmosphere
heating furnace 3. Thereafter, the strip passes about turn-around
roll 17 and continues downwardly again through furnace 3. The
controlled atmosphere heating furnace may be of the radiant tube
type and will further raise the temperature of the ferrous base
metal strip 9 to from about 650.degree. C. (1250.degree. F.) to
about 925.degree. C. (1700.degree. F.), depending upon the nature
of the ferrous base metal strip and the desired final
characteristics of the base metal strip.
The strip preparation furnace of the coating line 1 may have one or
more cooling chambers. For purposes of this exemplary showing, the
strip preparation furnace is illustrated as having two cooling
chambers 4 and 5. From the controlled atmosphere heating furnace 3
the strip 9 passes about turn-around rolls 18 and 19 and enters
cooling chamber 4. Chamber 4 may be of the tube cooling type well
known in the art. In the exemplary illustration the ferrous strip 9
makes three vertical flights through cooling chamber 4, passing
about turn-around rolls 20 and 21. Thereafter, the ferrous base
metal strip 9 passes about turn-around rolls 22 and 23 to enter the
second cooling chamber 5 which may be of the jet cooling type,
again well known in the art.
The temperature to which the ferrous base metal strip 9 is cooled
will depend upon a number of factors. Since the molten coating
metal 7 in coating pot 8 is zinc or zinc alloy, the ferrous base
metal strip will preferably be cooled to approximately 450.degree.
C. (840.degree. F.). In some instances, however, the strip itself
may be used as an additional means to introduce heat into the
molten coating metal bath 7. Under these circumstances the ferrous
base metal strip 9 may be introduced into bath 7 at a temperature
somewhat higher than the melting point of the zinc or zinc alloy
therein. Where the strip is not relied upon as one of the heat
sources for the bath 7, the strip may be introduced into the bath
at a temperature slightly below that of the bath. In any event, the
strip temperature should be sufficiently high as to prevent casting
of the molten coating metal thereon. By the same token, the strip
temperature must not be so high as to bring about excess coating
metal-base metal alloying.
From the cooling chamber 5 the ferrous base metal strip 9 passes
about turn-around roll 24 and enters the snout 6. It will be noted
that the free end of snout 6 extends downwardly below the surface
of the zinc or zinc alloy bath 7. The ferrous base metal strip
passes about turn-down roll 25 and is directed downwardly into bath
7. Within the bath, the strip is guided by one or more coating pot
rolls so as to exit in a substantially vertical flight. In the
embodiment shown, a single coating pot roll 26 is illustrated. The
two-side coated ferrous base metal strip 9a exits the molten
coating metal bath 7 and enters an enclosure 17, the lower end of
which extends into the molten coating metal bath 7 to form a seal
therewith. Within enclosure 27, the two-sided coated ferrous base
metal strip 9a is caused to pass between a pair of jet finishing
nozzles 28 and 29.
It will be noted from FIG. 1 that the upper end of the direct fired
furnace 2 is connected by a conduit 30 to an exhaust fan 31. The
outlet 32 of exhaust fan 31 may be connected directly to a stack or
to waste gas heat reclamation means (not shown). The strip
preparation furnace of coating line 1 is be operated above
atmospheric pressure (to prevent the introduction therein of oxygen
from the ambient atmosphere) by controlling the discharge rate of
the products of combustion from the direct fired furnace 2. To this
end, a damper 33 may be located in conduit 30. The parameters under
which the strip preparation furnace of coating line 1 is run do not
constitute a limitation on the present invention.
Reference is now made to FIG. 2 wherein the snout 5, coating pot 8
and enclosure 27 of FIG. 1 are shown enlarged. Like parts have been
given like index numerals. In the embodiment of FIGS. 1 and 2,
snout 6 and the enclosure 27 are illustrated as constituting wholly
separate structures. It will be understood by one skilled in the
art that the enclosure 27 could constitute an integral part of
snout 6. When a chemical and flux pretreatment system is used, the
snout 6 can be eliminated.
In accordance with the method of the present invention, a
non-oxidizing atmosphere is maintained within enclosure 27. Any
appropriate non-oxidizing or inert atmosphere may be used. A
nitrogen atmosphere is preferred as being the most economical. The
jet nozzles 28 and 29 can serve as the source of the atmosphere
within enclosure 27, although additional atmosphere inlets such as
the inlet 34 may be provided, if required.
A portion of the nitrogen atmosphere within enclosure 27 may be
withdrawn and recirculated through the jet finishing nozzles 28 and
29. This is diagrammatically illustrated in FIG. 2. The enclosure
27 is provided with an outlet 35. The outlet 35 is preferably
connected to a high temperature baghouse 35a, the atmosphere
withdrawn from enclosure 27 passes to a heat exchanger 36. The heat
exchanger 36 is connected as at 37 to the input 38 of a blower 39.
The purpose of the heat exchanger is to cool the nitrogen from
enclosure 27 ahead of blower 39 to prevent overheating of bearings
and seals in the blower. The baghouse 35a could be located between
heat exchanger 36 and blower 39, although it is preferred that it
be ahead of exchanger 36 to prevent clogging of the heat exchanger
fins with zinc or zinc oxide dust. The output 40 of blower 39 is
connected by conduits 41 and 42 to jet finishing nozzles 28 and 29.
The conduits or lines 41 and 42 may contain valves 43 and 44,
respectively, so that the plenum pressure of the jet finishing
nozzles 28 and 29 can be adjusted. It has been found that through
the use of such baghouse-heat exchanger-blower-sealed conduit
system, more than 50% of the high purity nitrogen requirement can
be recirculated from enclosure 27 through jet finishing nozzles 28
and 29, thus reducing the nitrogen consumption. Make-up nitrogen
may be introduced into the system via conduit 45 connected to the
line 37 between heat exchanger 36 and the intake 38 of the blower
39. The atmosphere recirculation rate is so adjusted as to avoid
infiltration of air through the slot 46 through which the coated
strip 9a exits enclosure 27. Such a recirculating system can be
used with any non-oxidizing or inert atmosphere.
The jet nozzles 28 and 29 are located to either side of the two
side coated base metal strip 9a and directly opposite each other,
as shown in FIG. 1. Edge problems including wrap-around have been
eliminated by the present method, and it is preferred that the jet
nozzles 28 and 29 be staggered vertically with respect to each
other as shown in FIG. 2. This greatly reduces noise and prevents
clogging of the nozzles due to zinc splatter and blow-off from one
nozzle to the other, as explained above. Either jet knife can be
located above the other. The higher of the two jet finishing
nozzles (in this instance jet finishing nozzle 28) can be located
up to about 2 feet (0.6 meters) or more above the bath. The jet
finishing nozzles 28 and 29 may be vertically offset with respect
to each other by any amount desired. Generally, they are offset
from about 1/4 inch (6 mm) to about 6 inches (150 mm). Usually the
nozzles will be as close to the strip as possible within practical
limits. For example, excellent results have been achieved with the
nozzles spaced from the strip by a distance less than 1 inch (25
mm). When the jet finishing nozzles are offset, the noise level of
the finishing step caused by the nozzles is greatly reduced. The
jet nozzles 28 and 29 are preferably of simple construction, having
a simple rectangular jet opening and being free of curved lips,
shutters, vanes, or other devices. Excellent results have been
achieved using jet finishing nozzles having a simple rectangular
opening with a uniform width in the range of from about 0.05 to
about 0.08 inch (1.3 to about 2 mm) throughout its length. Such jet
nozzles have been found to provide very uniform cooling of the
strip resulting in improved strip shape. This improved strip shape
(flatness) allows the above noted close spacing of the jet nozzles
to the strip, providing a greater flexibility in coating weight
control while using less finishing gas. Such jet nozzles also
provide a very uniform coating across the width of the strip, no
longer requiring a heavy coating at the center of the strip to
compensate for edge build-up.
The enclosure 27 is provided with an exit opening or slot 46 for
the two-side coated ferrous base metal strip 9a. Care must be taken
to assure that ambient air is not aspirated through the slot 46 due
to high gas velocities and turbulent effects operating within the
enclosure near the slot 46. Such turbulent effects may be
intensified by recirculation. Ambient air aspirated through the
slot 46 would cause excessive oxygen to be present in enclosure 27.
The use of baffles or additional nitrogen purging around the strip
exit 46 may assist in preventing such air aspiration. However,
excellent results have been achieved by simply providing a short
chimney 47 and locating exit slot 46 atop chimney 47.
The atmosphere within chamber 27 and the jet finishing gas are
non-oxidizing or inert and, as indicated above, nitrogen is
preferred for reasons of economy. For the very best results, the
atmosphere within the chamber 27 and the jet finishing medium
should have an oxygen content of less than about 100 ppm. Excellent
results are achieved at an oxygen content of less than about 200
ppm. As the oxygen content of either the chamber atmosphere or the
jet finishing medium increases above about 200 ppm, the benefits of
the present invention diminish. While there is no absolute upper
limit to the permissible oxygen content, and while at an oxygen
content of about 1000 ppm some benefits over prior art practice are
achieved, at an oxygen content above about 1000 ppm, the benefits
achieved would probably not justify the practice of the present
invention.
The enclosed finishing method of the present invention permits a
short immersion, shallow coating pot practice utilizing a partially
submerged coating pot roll. This is true because the method of the
present invention minimizes the formation of oxide on the surface
of the bath.
FIG. 3 illustrates such a short immersion, shallow coating pot
practice. In FIG. 3 a coating pot is shown at 48. The coating pot
48 is similar to coating pot 8 of FIG. 2 with the exception that it
is shallower. The coating pot 48 contains a bath of molten coating
metal 49 which is of considerably less volume than the bath 7 of
FIG. 2.
A snout 50, equivalent to snout 6 of FIG. 2, is shown with its
lowermost end located beneath the surface of bath 49 so as to be
sealed thereby. The snout 50 contains a turn-down roll 51
equivalent to turn-down roll 25 of FIG. 2. A pot roll is
illustrated at 52. The pot roll 52 differs from pot roll 26 of FIG.
2 in that it is only partially submerged in the molten coating
metal bath 49. The apparatus of FIG. 3 includes an enclosure 53
which is equivalent in every way to enclosure 27 of FIG. 2 with the
exception that its lower rear edge 53a is bent slightly downwardly
and inwardly so as to make a seal with the molten coating bath 49
while at the same time providing clearance for the flight of the
uncoated ferrous base metal strip 54 between turn-down roll 51 and
the pot roll 52. The coated ferrous base metal strip 54a is shown
passing between a pair of jet finishing nozzles 55 and 56 and
upwardly through a chimney 57 and an exit slot 58 equivalent to
chimney 47 and exit slot 46 of FIG. 2.
The operation of the coating and finishing apparatus shown in FIG.
3 is substantially identical to that described with respect to FIG.
2. Again, jet finishing nozzles 55 and 56 may be connected to a
recirculating system (not shown) of the type illustrated in FIG. 2.
The primary difference between the operation illustrated in FIG. 3
and that illustrated in FIG. 2 lies in the fact that pot roll 52 is
only partially submerged which provides the above noted
advantages.
The amount by which pot roll 52 is submerged in bath 49 can be
varied. In FIG. 3 pot roll 52 is shown more than half submerged.
With appropriate configuration of snout 50 and the portion 53a of
enclosure 53, the pot roll 52 could be less than half submerged,
particularly in those instances where it is desirable to maintain
the roll bearings (not shown) above the bath surface.
The pool of molten metal 59 between pot roll 52 and the ferrous
base metal strip 54 engaging the pot roll must be of sufficient
size to assure adequate coating of the back side or roll side of
the ferrous base metal strip. It will be understood that the size
of the pool 59 will decrease as the amount by which pot roll 52 is
submerged decreases. It is within the scope of the present
invention to augment this situation through the use of a grooved
pot roll 52 or means to pump additional molten coating metal into
the pool 59 (as will be described hereinafter).
FIG. 4 illustrates another arrangement to assure adequate coating
of the backside or roll side of the ferrous base metal strip in
shallow pot practice. In FIG. 4 an enclosure is illustrated at 60
which may be identical to the enclosure 27 of FIG. 2, having a pair
of jet finishing nozzles 61 and 62, an exit chimney 63 and an inlet
64 for the inert or non-oxidizing atmosphere. It will be understood
that the enclosure 60 may be provided with the atmosphere
recirculation system described with respect to FIG. 2. The lower
end of enclosure 60 is submerged in bath 65 of molten coating metal
located in a shallow pot 66. FIG. 4 also illustrates a conventional
snout 67, similar to snout 6 of FIG. 2. Again it will be noted that
the lowermost end of snout 67 extends below the surface of the
molten coating metal bath 65.
The ferrous base metal strip to be coated is shown at 68. The strip
passes about turn-down roll 69 in snout 67 and enters the bath as
it passes about a first pot roll 70. From pot roll 70 it extends to
a second pot roll 71 which directs the coated strip 68a upwardly
through enclosure 60.
The amount by which pot rolls 70 and 71 extend into the molten
coating bath 65 can be varied. For purposes of this exemplary
showing, pot rolls 70 and 71 are illustrated as extending into the
molten coating metal bath 65 by an amount less than 1/2 their
diameters. The existence of the submerged strip 68b between pot
rolls 70 and 71 assures adequate coating of the backside or roll
side of the ferrous base metal strip. It has been determined that
the shallow pot practice of the type just described with respect to
FIGS. 3 and 4 does not significantly change the enclosed nitrogen
finishing characteristics or advantages described with respect to
FIGS. 1 and 2.
As has already been made evident, the apparatus of the present
invention may utilize various pot roll arrangements. Another
arrangement is illustrated in FIG. 5. In this Figure, a
conventional coating pot is shown at 72 containing a molten coating
metal bath 73. A snout 74, equivalent to snout 6 of FIG. 2 has its
lower end submerged in the molten coating metal bath 73 and is
provided with a turn-down roll 75, equivalent to turn down roll 25
of FIG. 2. An enclosure 76 has its lower end submerged in the
molten coating metal bath 73. The enclosure 76 may be identical to
enclosure 27 of FIG. 2, having an exit chimney 77 and an atmosphere
inlet 78, if required. The enclosure contains a pair of jet
finishing nozzles 79 and 80 equivalent to jet finishing nozzles 28
and 29 of FIG. 2. Again, the enclosure 76 may be provided with the
atmosphere recirculating system (not shown) of FIG. 2.
In this embodiment, the ferrous base metal strip 81 to be coated
enters the molten coating metal bath 73 and passes about a series
of three pot rolls 82, 83 and 84. Rolls 83 and 84 are stabilizer
rolls and provide strip shape control, assuring flatness of the
coated strip 81a as it passes between jet finishing nozzles 79 and
80.
In all of the embodiments thus far described, the enclosure and the
snout have been illustrated as separate structure. It is also
within the scope of the present invention, however, to provide a
snout and an enclosure which constitute an integral, one-piece
structure. This is illustrated in FIG. 6.
In FIG. 6 a conventional coating pot 85 is shown containing a bath
86 of molten coating metal. The snout-enclosure structure is
generally indicated at 87, having a snout portion 87a and an
enclosure portion 87b. The snout portion 87a is similar to snout 6
of FIG. 2 and has a turn-down roll 88 located therein. Turn-down
roll 88 is equivalent to turn-down roll 25 of FIG. 2. The enclosure
portion 87b is similar to enclosure 27 and has an exit chimney 89.
The enclosure portion 87b may be provided with an atmosphere inlet
90 equivalent to inlet 34 of FIG. 2. Jet knives 91 and 92 are
located within the enclosure portion 87b and are in every way
equivalent to jet knives 28 and 29 of FIG. 2. It will further be
understood that the enclosure portion 87b may be provided with an
atmosphere recirculating system (not shown) equivalent to that
described with respect to FIG. 2. In the embodiment of FIG. 6, a
submerged pot roll is shown at 93.
Under normal circumstances, the snout portion 87a and the enclosure
portion 87b will contain different atmospheres and therefore some
sort of seal means should be provided therebetween. The seal means
may take any appropriate form. For purposes of an exemplary
showing, the seal means is illustrated as being made up of two
pairs of sealing rolls 94-95 and 96-97.
It is within the scope of the invention to provide an inlet 98 for
an appropriate non-oxidizing gas between sealing rolls 94-95 and
sealing rolls 96-97. It is preferable that the non-oxidizing
atmosphere between sealing rolls 94-95 and sealing rolls 96-97 be
at a pressure slightly higher than the pressure of the atmosphere
in snout portion 87a and enclosure portion 87b. This assures that
either the enclosure portion 87b or the strip preparation furnace
associated with snout 87a can be shut down without contaminating
the other. It will also prevent contamination of the atmosphere
within hood portion 87b from sources at the entry end of the
conventional strip preparation apparatus.
The strip 99 to be coated passes about turn-down roll 88 and
between sealing roll pairs 94-95 and 96-97. The strip 99 enters the
bath and passes about pot roll 93. Thereafter, the coated strip 99a
passes upwardly between jet finishing nozzles 91 and 92, exiting
through exit chimney 89. Thus, the operation of the apparatus and
the advantages achieved thereby are essentially the same as has
been described with respect to FIGS. 1 and 2.
The unitary snout-enclosure of FIG. 6 can also be applied to
shallow pot practice. This is illustrated in FIG. 7. In FIG. 7, the
snout-enclosure apparatus is identical to that of FIG. 6 and like
parts have been given like index numerals. In the embodiment of
FIG. 7, a shallow pot is shown at 100, containing a shallow bath
102 of molten coating metal. In this instance, a pot roll 103 is
shown being partially submerged in the molten metal coating bath
102. For purposes of this exemplary showing, the pot roll is
illustrated as being submerged by an amount less than half its
diameter. The pot roll 103 could, of course, be submerged by an
amount more than half its diameter, as is shown with respect to pot
roll 52 of FIG. 3. It would even be possible to provide the
apparatus of FIG. 7 with a pair of pot rolls of the type described
with respect to FIG. 4.
In FIG. 7, however, for purposes of an exemplary showing the
apparatus is illustrated as being provided with a pump for the
molten coating metal of bath 102, the outlet of the pump being
shown at 104. The pump outlet 104 creates a pool 105 of molten
coating metal between the ferrous base metal strip 99 and pot roll
103, which pool assures adequate coating of the back or roll side
of the ferrous base metal strip. Such a pump for the molten coating
metal could be provided for the embodiment of FIG. 3, if the pool
59 of FIG. 3 were inadequate. In all embodiments of the present
invention, where the pot roll is only partially submerged, it would
be within the scope of the invention to use a grooved pot roll. The
grooves carry molten coating metal to the roll side of the ferrous
base metal strip.
With the method of the present invention, it has been found that
relatively heavy coatings can be achieved at lower line speeds with
excellent surface characteristics. For example, with minimum
controlled wipe by the jet finishing nozzles, coating weights of up
to about 543 g/m.sup.2 (about 1.78 ounces per square foot) have
been achieved at a line speed of 40 feet per minute (12 meters per
minute) in the laboratory.
A laboratory test found that the finishing method of the present
invention produced a coated strip with extremely flat spangle. The
spangle boundary relief was so slight that it was not necessary to
practice spangle minimizing techniques to achieve excellent surface
quality. In commercial, in-plant runs, it was found preferable to
practice a spangle minimizing step, the results of which produced
an excellent surface suitable for the application of paint for
automotive and appliance uses. The results of the minimizing step
were not only excellent, but also were consistent, and required
less operator adjustment of the minimizing equipment than
previously experienced when finishing with air.
Any of the above mentioned minimizing processes can be used. While
not intended to be limiting, a preferred minimizing procedure is
taught in the above mentioned U.S. Pat. No. 3,379,557, the
teachings of which are incorporated herein by reference. Briefly, a
water solution of an inorganic salt is mixed with steam and sprayed
against the freshly coated strip at a point just below that
position where normal coating solidification would occur. The
inorganic salt is selected from the class consisting of inorganic
salts which decompose in the range of 175.degree. F. to 550.degree.
F. (80.degree. C. to 90.degree. C.) and those salts which will
hydrolize when added to water to form inorganic salts capable of
decomposing in the above stated temperature range. The water
solution is applied to the coated strip in a band extending
transversely of the direction of strip travel, the band having such
a width that the coating metal is molten as it enters the the band
and solid as it leaves the band. The inorganic salt solution of the
type described provides a multitude of solidification nuclei to the
coating when the coating is at a temperature very close to the
solidification point (or freezing point) of the coating metal. This
results in inducing a multitude of closely spaced, relatively
minute, spangles which are sub-microscopic, or so nearly so as to
be just barely visible to the naked eye.
This procedure is diagrammatically illustrated in FIG. 1. The
two-side coated ferrous base metal strip 9a exits chamber 27 and is
caused to pass between a pair of tray-like structures 106 and 107
and through an enclosure 108 containing spray nozzles 109 and 110
for the minimizing inorganic salt solution. Trays 106 and 107 serve
to catch the majority of the overspray condensate. It will be
understood that this or another appropriate minimizing process can
be practiced with all of the embodiments of the present invention
illustrated in FIGS. 2 through 7.
The consistently excellent surface achieved in the plant runs is a
direct result of the finishing method of the present invention.
This is true because the finishing method of the present invention
provides a more uniform coating across the width of the ferrous
base metal strip; a more uniform cooling of the coated strip
edge-to-edge results in a more uniform thermal profile; and less
surface oxide to interfere with nucleation. The more uniform
cooling and reduced amount of surface oxide produced by the
finishing method of the present invention gives more latitude to
the placement of the minimizing nozzles 109 and 110 and reduces the
amount of spray required. With less spray there is less hazard of
coating damage from pitting. A smaller, more uniform spangle size
is consistently achieved across the strip width. In instances where
spangle is of no concern or is desired, and in instances where a
coating metal other than zinc or zinc alloy is used, a minimizing
step need not be practiced.
The present invention has been practiced with aluminum as the
coating metal and a smoother coating with no edge build-up was
produced.
EXAMPLE 1
A laboratory galvanizing line utilizing a 4 inch (100 mm) strip was
provided with an enclosure similar to enclosure 27 of FIG. 2. The
chimney 47 was six inches (150 mm) high and was provided with an
exit slot 46 having a width of one and one quarter inches (30 mm)
and a length of five inches (130 mm). The enclosure was provided
with a pair of jet finishing nozzles having a slot-like opening of
0.050 inch (1.3 mm) width throughout its length. The lower of the
two jet finishing nozzles was maintained at a distance of four
inches (100 mm) from the bath surface. The other jet nozzle was
offset vertically and upwardly therefrom by one half inch (13 mm).
The jet nozzles were maintained at a distance from the strip of
about one quarter inch (6 mm). The enclosure was provided with a
recirculating system of the type shown in FIG. 2. Make up nitrogen
was added at the rate of 3,000 cubic feet per hour (84 cubic meters
per hour) and the nitrogen atmosphere within the enclosure was
maintained at a pressure of 0.5 inches (13 mm) of water.
The cold rolled ferrous base metal strip gage was 0.015 inches (0.4
mm) with relatively smooth surfaces at 50 inches (1.3 meters) and
90 peaks per inch (3500 peaks per meter). During this run, a G-60
coating was being produced and a line speed of 70 feet per minute
(21 meters per minute) was used. The influence of oxygen
contamination in the enclosure containing high purity nitrogen was
evaluated by metering compressed air into the recirculating system
in increasing amounts until defects were observed in the molten
coating. With oxygen below 50 ppm the molten coating was glassy
smooth, free of visible oxide and without sign of edge problems.
The solidified coating showed a dead flat spangle without spangle
boundary relief. As the oxygen was purposely increased, no change
occurred at an oxygen level of 140 ppm. Detrimental finishing
effects were first observed at an oxygen level within the enclosure
of about 200 ppm in the form of edge oxide berries, ripples, a
ridge of heavy edge metal, and some spangle relief. These
conditions become steadily more pronounced as the oxygen level was
increased to 600 ppm. Surface oxide bands developed when the oxygen
level reached about 700 ppm. These oxide bands extended inwardly
from the strip edge and increased to feathers of oxide when the
oxygen level reached 850 ppm.
This run showed that the enclosed nitrogen finishing method of the
present invention produces a smooth, uniform hot-dip zinc coating
finish without the common ripple, dross, oxide curtain and edge
build-up defects associated with conventional edge finishing.
Simplified jet finishing nozzles with uniform slot openings can be
used and can be vertically offset without zinc splatter and without
heavy edge coating or coating wrap-around. The noise level of the
finishing step was drastically reduced by virtue of the fact that
the nozzles were offset with respect to each other. The
relationship between oxygen contamination of the finishing gas and
the coating surface quality was clearly demonstrated. The above
noted comments regarding the effects of oxygen contamination of the
finishing gas, demonstrated by this experimental laboratory run,
are made comparing the coated metal at various oxygen levels at and
above about 200 ppm. Excellent results are achieved when the oxygen
level within the enclosure is maintained below about 200 ppm and
preferably below about 100 ppm. As the oxygen level increases from
about 200 ppm to about 1000 ppm detrimental effects such as oxide
berries, edge build-up and spangle relief increase. Even within
this range, however, the coating quality is superior to that
achieved by the prior art practice of finishing in air.
In other similar runs nitrogen at the rate of 3000 cubic feet (84
cubic meters) per hour was circulated through the jet finishing
nozzles using about 1500 cubic feet (42 cubic meters) per hour or
less make-up nitrogen, confirming the ability to recirculate more
than 50% of the high purity finishing gas requirement.
EXAMPLE 2
An in-plant, commercial line was provided with an enclosure similar
to enclosure 27 of FIG. 2. A G60 zinc coating was applied to a
0.031 inch (0.8 mm) ferrous base metal strip having a width of 60
inches (1.5 meters) and a line speed of 240 feet (73 meters) per
minute. The enclosure was provided with a pair of jet finishing
nozzles, each having a slot-like opening 0.070 inch (1.8 mm) wide
and 84 inches (2 meters) long. The front nozzle was positioned 14
inches (0.35 meters) above the bath surface and the back nozzle was
located 133/4 inches (0.34 meters) above the bath surface. Both
nozzles were oriented normal to the strip and spaced therefrom by
1/2 inch (13 mm). A nitrogen flow of 50,000 cubic feet (1375 cubic
meters) per hour was maintained in each nozzle with no nitrogen
recirculation. An oxygen content of 73 ppm was maintained in the
enclosure. The coated and finished strip was subjected to a spangle
minimizing step in accordance with the above mentioned U.S. Pat.
No. 3,379,557.
EXAMPLE 3
Another in-plant run was made on the same commercial line used in
Example 2. A G60 zinc coating was applied to a 0.053 inch (1.4
meters) ferrous base metal strip having a width of 55 inches (1.4
meters) and traveling at a speed of 200 feet (60 meters) per
minute. The remaining steps and parameters wer the same as in
Example 2 with the exception that the jet finishing nozzles were
both spaced 3/4 inch (20 mm) from the strip and each has a nitrogen
flow therethrough of 40,000 cubic feet (1000 cubic meters) per
hour. The oxygen content within the enclosure was maintained at 80
ppm. The spangle minimizing step was performed.
The material of Example 2 had a coating weight of 0.32 ounces per
square foot (98 grams per square meter) on its front side and a
coating weight of 0.33 ounces per square foot (101 grams per square
meter) on its back side. The material of Example 3 had front and
back coating weights of 0.33 and 0.31 ounces per square foot (101
and 95 grams per square meter), respectively. The materials of both
Example 2 and Example 3 were characterized by excellent coatings
which were smooth, uniform, and free of oxide defects and edge
build-up. Superior and consistent spangle minimization was
achieved, rendering the coated materials suitable for critical
surface uses.
Laboratory runs have been made using aluminum as the coating metal.
Again, smooth, uniform coatings free of edge build-up and oxide
defects were achieved. This is true despite the fact that, in
general, aluminum coatings are more subject to dross and ripple
defects than zinc coatings.
In all of the embodiments illustrated in the Figures, the enclosure
is shown in semi-diagrammatic fashion. It will be understood by one
skilled in the art that the enclosure will be provided with
suitable support means and the like. Furthermore, the enclosure may
be removable in whole or in part for maintenance or if regular air
finishing is to be practiced.
The products of the present invention can be subjected to any
appropriate and well known past treatments such as special
treatments for better paint adherence and the like.
Modification may be made in the invention without departing from
the spirit of it.
* * * * *